Autosomal dominant ectodermal dysplasia with immune deficiency (AD ED-ID) is characterized by sparse hair, conical teeth, reduced number of sweat glands and susceptibility to severe infections. It is due to heterozygous mutations in I?B? that impair Ser32 and Ser26 phosphorylation and subsequent degradation of the mutant protein following receptor signaling. The non-degraded mutant protein accumulates and sequesters the p50 and p65 subunits of NF?B in the cytoplasm, impairing activation of the canonical NF?B pathway through a dominant negative (DN) effect. Our genotype-phenotype analysis of 9 patients with AD ED-ID indicates that disease severity and impairment of NF?B-dependent TLR-driven cytokine production are significantly worse in patients with I?B? point mutations than in those with N-terminal truncations. We hypothesize that the point mutants may be more stable than the truncation mutants, and thereby accumulate at a higher level, resulting in a stronger DN effect and more severe disease. Our analysis also shows that patients with AD ED-ID have a poor outcome after hematopoietic stem cell transplantation (HSCT) despite good donor chimerism. We have created a knock-in mouse heterozygous for the I?B? S32I mutation that recapitulates the phenotype of patients with AD ED-ID, The S32I mouse fails to develop contact hypersensitivity (CHS) or produce antibodies. Strikingly, the mutant completely lacks secondary lymphoid organs including lymph nodes and Peyer's patches, and fails to form germinal centers (GCs), two features typical of defective non-canonical NF?B signaling that were not previously recognized in AD ED-ID patients. LT?R-driven induction of chemokines and adhesion molecules mediated by both canonical and non-canonical NF?B pathways was impaired, and the levels of p100, a component of the non-canonical NF?B pathway, were markedly diminished in the mutant. I?B? mutant->Rag2-/-, but not WT->I?B? mutant, bone marrow chimeras formed proper lymphoid organs, developed CHS and formed GCs. These results suggest that defective architectural cell function underlies the immunodeficiency and poor outcome of HSCT in patients with I?B? mutations, and that correction of this niche may be critical for reconstituting their immune function. We propose to test the hypothesis that the stability of the I?B? mutant protein inversely correlates with the residual NF?B activity and th level of p100 in the patient cells, and thus determines disease severity and the outcome of HSCT. We will test the hypothesis that mesenchymal stem cells (MSCs) from mice with the S32I I?B? mutation have reduced p100 levels and impaired response to LT?R ligation, and to evaluate implantation of WT MSCs as a potential therapeutic strategy in a preclinical model for AD ED-ID.